1. Field of the Invention
This invention relates to aromatization of alkanes having one to four carbon atoms per molecule to aromatics, such as benzene, toluene and xylenes (BTX) using a catalyst including a crystalline zeolite, specifically a platinum-containing ZSM-5.
2. Description of the Prior Art
Aromatization is a well-known reaction wherein alkanes are converted to aryls. Aryls, such as benzene, toluene and xylene, can be commercially produced by catalytic reforming of petroleum naphtha. However, naphtha is in great demand for other petrochemical products, such as gasoline.
One example of an aromatization process which does not use naphtha as a feedstock is the Cyclar™ process which converts liquefied petroleum gas (LPG) directly into a liquid aromatics product in a single operation. LPG consists mainly of propane and butane but can also contain C2, C5 and C6 alkanes and C2–C6 olefins. LPG, which is primarily recovered from gas and oil fields and petroleum refining operations, is relatively low in value and is available in abundance, qualities which make it a good feedstock for petrochemical applications, such as aromatization.
The Cyclar process is described as dehydrocyclodimerization, which is a sequential dehydrogenation of C3 and/or C4 alkanes to olefins, oligomerization of the olefins, cyclization to naphthenes and dehydrogenation of naphthenes to corresponding aromatics. Hydrocracking side reactions of the olefins and oligomers generate methane and ethane. The dehydrogenation reactions generate hydrogen. The typical catalyst used in this process is a gallium containing ZSM-5 zeolite.
U.S. Pat. No. 5,672,796 discloses a one stage process for aromatization of C3–C6 saturated hydrocarbons to a low methane-containing aromatic product using a partially-sulfided platinum/rhenium-containing crystalline aluminosilica molecular sieve catalyst with MFI crystal structure, preferably with a Si/Al ratio between 40 and 600.
U.S. Pat. No. 4,788,364 discloses a two stage process for dehydrocyclization and oligomerization-aromatization of C2–C10 paraffins to highly aromatic and olefinic gasoline using crystalline aluminosilicate catalysts, such as ZSM-5, which may incorporate phosphorus, gallium, tin, rhenium, zinc, platinum and copper. There were no examples in which these metals were incorporated into the catalyst and no disclosure of suppressing methane.
U.S. Pat. No. 4,835,336 discloses a process for converting nonaromatic C6+ hydrocarbons to aromatics with a Pt/ZSM-5 catalyst which has been presulfided to increase aromatic selectivity and suppress hydrogenolysis, i.e., methane formation is passivated. There is no disclosure of use of such a catalyst, with or without presulfiding, with lower alkanes.
U.S. Pat. No. 4,766,265 discloses a catalyst for the conversion of ethane to liquid aromatic hydrocarbons. The catalyst is a zeolite-type alumino-, gallo- or boro-silicate (ZSM-5 or ZSM-11 being preferred) which has gallium, aluminum and/or zinc incorporated into the structure and has been treated with rhenium and a metal selected from nickel, palladium, platinum, rhodium and iridium, with platinum and rhodium being preferred. The second metal was believed to act as a promoter for the dehydrogenation reaction of the oligomerized intermediates formed from ethane which enhanced the overall selectivity to aromatics and decreased methane selectivity promoted by rhenium.
U.S. Pat. No. 3,756,942 discloses a process for the preparation of aromatic compounds from a feed of a mixture of paraffins, olefins and naphthenes with a ZSM-5 catalyst. The examples disclose a platinum-impregnated H-ZSM-5 catalyst which converted hexane to aromatics. Weight per cent of platinum varied from 0.04 to 0.35% and aromatic selectivity varied from 29 to 32.
U.S. Pat. No. 3,760,024 discloses a process for preparation of aromatic compounds from C2–C4 paraffins or olefins with a ZSM-5 type catalyst. A hydrogenation/dehydrogenation component, such as metals and oxides and sulfides of metals of Group VIB, Group IIB, Group VIIB and Group VIII, is not necessary but may be present.
U.S. Pat. No. 5,827,422 discloses a catalyst for converting a hydrocarbon or a hydrocarbon mixture to an olefin and a C6 to C8 aromatic hydrocarbon. The composition is an aluminosilicate of a silica, an alumina, and, optionally, platinum wherein the weight ratio of elemental aluminum to elemental silicon is in the range of from about 0.002:1 to about 0.25:1 and the weight ratio of the optimal platinum to silicon is in the range of from about 0.0005:1 to about 0.01:1. The hydrocarbon may be butane, isobutanes, pentane, isopentane, hexane, isohexane, cyclohexane, heptane, isoheptane, octane, isooctane, nonane, decane, undecane, dodecane, tridecane, tetradecane, pentadecane, hexadecane, butenes, isobutene, pentenes, hexenes, benzene, toluene, ethylbenzene, xylenes, and combinations of any two or more thereof and is preferably gasoline or a gasoline derivative.
U.S. Pat. No. 6,017,422 discloses a catalyst composition which is resistant to sulfur or a sulfur compound containing a zeolite, cerium or cerium oxide, and a Group VIII metal or metal oxide, such as platinum or platinum oxide, for converting hydrocarbons to C6–C8 aromatic hydrocarbons. The hydrocarbons may be those having 1 to about 30 carbon atoms, preferably aliphatic saturated hydrocarbons, such as ethane, propane, butane, pentane, hexane, heptane, octane, nonane, dodecane, gasoline, or combinations of two or more thereof. The zeolite is any commercially available zeolite which can catalyze the conversion of a hydrocarbon to an aromatic compound, but is preferably L zeolite. The examples disclose the advantages (higher feed conversion, higher benzene yield, and higher selectivity to benzene in the presence of a sulfur compound) of a Pt/Ce-promoted zeolite L catalyst over a Pt-promoted zeolite L catalyst (without Ce).
In the article “Ethane to Aromatic Hydrocarbons: Past, Present, Future” by Anke Hagen and Frank Roessner, Catal. Rev.—Sci. Eng., vol. 42(4), page 403–437 (2000) the introduction of platinum into a H-ZSM-5 led to a 1.5–2.0 fold increase of activity of ethane aromatization but selectivity to aromatics was relatively low.
U.S. Pat. No. 6,593,503 discloses platinum-promoted ZSM-5 zeolite, acid-treated and not acid-treated, for aromatization of hydrocarbons, preferably 4–25 carbon atoms and more preferably “gasoline-type” hydrocarbons, to C6 to C8 aromatics, such as benzene, toluene and xylenes. The examples demonstrated the benefit of acid treating to reduce the aluminum content in the zeolite.
Because of the acidity inherent to zeolites alkane cracking occurs in an alkane aromatization process which results in producing undesirable methane. It would be advantageous if methane production could be suppressed in favor of producing more ethane relative to methane from the side reactions of alkane aromatization generated could be used.